Alkyl aryl ethers are excellent solvents for organic residues and are particularly good for dissolving the resinous or varnish-like deposits formed in the crankcase of internal combustion engines. They also are used as antioxidants, heat-transfer agents, and ingredients in perfumes. There seems to be no efficient and economical method of their preparation in commercial quantities. The Williamson synthesis appears to be the most important method and involves the reaction between an alkali metal salt of a hydroxy aromatic compound, that is, a phenol, and an alkyl halide or sulfate ester. Yields generally are low and there is a disposal problem for the alkali metal salt by-products.
Another approach to their preparation employs acid catalyzed addition of phenols to olefins. Generally this requires a strong acid and is conducted under heterogeneous conditions requiring separation of the acid. Where a solid phase catalyst is used, for example, aluminas and cationic exchange resins, the favored reaction usually is not O-alkylation to afford the alkyl aryl ethers, but is instead C-alkylation to afford ring-substituted phenols.
This precise problem was addressed by the patentee of U.S. Pat. No. 4,299,996 who observed that a fluorosulfonic acid resin was an effective catalyst for O-alkylation, whereas unmodified sulfonic acid resins themselves were not suitable for O-alkylation. Presumably this difference arose from the added acidity attributable to the strongly electron withdrawing fluorine atoms adjacent to the sulfonic acid group. Expressed differently, an influence from the aforementioned patent is that a particularly strong acidic resin is a prerequisite for its successful use as a catalyst in O-alkylation of phenols by olefins.
In view of the above, it was quite surprising to discover that ordinary cationic exchange resins may be successfully used as catalysts in the O-alkylation of phenols by olefins. An even more unexpected discovery is that when such resins are partially neutralized, that is, when they are partly in the form of their salts, they also are effective as O-alkylation catalysts, although under substantially different conditions than when used in the acid form.